Electroacoustic transducer



Sept. 28, 1943.

F. R. EARROW, JR

I 2,330,697 ELECTRO-ACOUSTIIC TRANSDUCER Filed April 2, 1942 llmllllllmllll T fmenfiwx- L III/I d I Sept. 28, 1943. F. R. FARRow, JR 2,330,697

ELECTRO-ACOUSTIC TRANSDUCER Filed April 2, 1942 2 Sheets-Sheet 2 IPESPO/YSE {vans} I00 zoaa /o. aao FREQUENCY OF 5TYL/l6 asc/LLnT/o/Y I jmerzlo i x- Patented Sept. 28, 1943 UNITED, STATES PATENT OFFICE Frederick R. Farrow, Jr., Audubon, N. 1., assignmto Philco Radio and Television Corporation, corporation of Delaware Application April 2,4942, Serial No. 437,430 1': Claims. (oi. ire-100.41)

Philadelphia, Pa., a

This invention relates to electro-acoustic transducers of the type adapted, when actuated by an acoustic system such as a phonograph recording, microphone or like device, to control an electrical system, such as a wave signal generator or amplifier.

More particularly, the invention is directed to a novel type oi phonograph pickup which utilizes a variable condenser directly operable by the usual stylus.

As disclosed in my prior Patent No. 2,322,621, granted June 22, 1943, such a transducer essentially consists of closely-spaced, insulated flexible metallic members forming a condenser, and means including a vibratory stylus member adaptedto torsionally vibrate said members to vary the capacity of said condenser, and thereby produce a signal corresponding to a recorded signal.

Such a device presents many advantages over transducers heretofore in use, among which are the following. It proyides an inexpensive means for frequency-modulating a radio frequency carrier wave, which means utilizes only a small power input, requires 'no critical adjustment; and is very stable in performance, being unafiected by changing atmospheric conditions.

In one form or such a transducer, shown herein, additional objects and advantages are attained, as will be best understood from an analysis of the inherent characteristics of this type of device- It should be first noted that it is inherently an amplitude-responsive device (i. e., it is responsive to the amplitude of the wave signal recorded on the record) and as such does not obtain the highest possible fidelity when used with modern recordings, as records at present available to the public are of a modified constant velocity type. They are recorded at constant amplitude over a relatively narrow low frequency range, say up to 300 C. P. 5., but are recorded at constant velocity over the greater part oi the audio range,i. e., irom the 300 cycle turnover irequency up to the upper recorded limit, which may be as high as 8000 cycles or more, depending upon the excellence oi the recording equipment. As is well known, over the constant velocity portion of the frequency range the amplitude of the recorded wave, and thereby oi the stylus displacement, falls oil by a ratio of 2 to 1 for every octave. Thus at 1000 C. P. S. the amplitude is only half that at 500 C. P. 8. Since the output of the system shown in my copending application depends upon variation oi the capacit oi a condenser in as 'ly high in volume, due

accordance with the lateral deflection of the stylus, it will be apparent that the output driving the amplifier will become increasingly smaller as the higher audio frequencies are approached. The bass notes are accordingly disproportionateto the attenuation of the high tones.

In addition, some units constructed in accordance with the broad invention, may at times show undesirable high level points throughout their audio response. These may arise from "break-up? of the condenser plates, that is, the condition in which one or more areas of a plate will start to resonate independently from the system as a whole. This can have the eflect oi sharply increasing'the vibrational amplitude, and thereby undesirably increasing the capacity change of the condenser.

With the above in mind, it is the primary ob- Ject oi the present invention to provide an improved amplitude-responsive capacity-type transducer of such construction as to obtain improved fidelity of reproduction of constant velocity recordings.

More particularly, the invention has as an object the provision pacity-type phonograph pickup, the plates of istlc existing in the constant velocity portion of the recording; and in which novel means is provided for damping the plate system in the operating frequency range.

Still more particularly, the invention provides a pickup of the abovecharacter, having condenser plates of tapered form, and having means for increasing the rigidity of the plates thereby maintaining the plates in intimate contact with the dielectric.

The invention also has as an object the provision of readily adjustable means for tuning, or trimming the condenser so as to sharplylimit the high frequency response at any predetermined point, and thereby eflectively control needle scratch by sharp cutofl oi the response.

It is alsoan object of this invention to provide a condenser, the plates of which have a very low moment of inertia around the axis of torsion, thereby improving the high frequency response of the pickup.

Other objects and advantages will be apparent hereinafter from the following description and the accompanying drawings, in which:

Fig. 1 is a side elevationv or a phonograph tonearm adapted for use in the reproduction oi disc of an amplitude-responsive ca- 7 Fig. is a perspective view (with the frame members broken away) illustrating certain modifications'of the presentinvention;

Fig. 5a is a transverse sectional view of one of the outside plates shown in Fig. 5;

Fig. 6 is a plan view of one of the condenser plates constructed according to this invention;

Fig. 7 shows, in side elevation, the shape of the plates before they are clamped; and

Fig. 8 is a graph showing the response characteristic of a pickup constructed in accordance with the present invention, as compared with the response of a pickup employing uniform rectangular plates.

Referring more particularly to the drawings in Fig. 1 there is shown a tone-arm I adapted to be pivotally mounted adjacent a turntable (not shown) for' movement thereover during the reproduction of a recording. A vertical post 2 may serve as the tone-arm pivotal support. At its free end arm -I has an enlarged head portion 3 within which the reproducing device, generally indicated at 4, is secured by suitable fastening means 5. 'I'heconstruction of the device 4 according to the present invention, will be described later.

In order more clearly to describe the present invention, there is illustrated in Fig. 2 a capacitytype pickup of the form disclosed in the abovementioned copending application. Referring to Fig. 2, the upper and lower halves 8 and I of a frame are shown apart so as to reveal the condenser members. Each part 6 and I has a rectangular'central opening 8, a flat sided front bearing yoke part 9 and .a flat rear clamping part ID. The yoke parts 9 form a rectangular or square bearing. The operative member'sof the to (see Fig. 1), and the assembled reproducer is mounted on the tone arm by bolts 5.

When the parts are assembled, as above described, the arm I6 normally extends substantially downward and clamp I3 is positioned at an acute angle with respect to the rear yoke parts I0 of frames 6 and I, so that the ribbons II and I2 are normally twisted about their longitudinal axes as illustrated. These ribbons may be permanently deformed or they may be retained in their twisted position'under initial stress, but in the form shown in Fig. 2 the ribbons may be placed under initial stress, and

maintained in a desired angular position bymeans of the locking of bushing I8 i'n yoke parts 9. Electrical contacts I9 and 20 may be secured to ribbons II and I2 and wires 2| may extend from these contacts through the tone arm to suitable circuits.

- The above described reproducing element constitutes an electrical condenser composed of the two ribbons II and I2 spaced by the insulator or dielectric Ho, and when the plates are in their normal position, as shown, they will constitute a condenser of predeterminedcapacity.

As the stylus I1 traverses the sound groove of a recording, the crank arm I6 will translate the lateral displacement of the stylus'by the record into an oscillatory motion of shaft I5. By virtue of. this, the ribbons II and I2 will be twisted or torsionally flexed about their longitudinal axes, the oscillations of arm I8 tending, when the arm is moving in one direction, to increase the twist of the ribbons, and when moving in the other direction, to decrease their twist. This movement of the ribbons will cause them to move toward and awayfrom each other throughout their length by a very small amount, thus changing the spacing of the ribbons, it being understood that they are not clamped together ex- 7 cept at their ends. This movement -of ribbons III and II relative to each other will, of course,

vary the capacity of the condenser in accordance:

with the lateral deflection of the stylus by recording.

In Fig. 3 there is shown by way-of exampleone suitable electrical circuit for use with the reproducing element above described, as well :as

reproducer comprise .two metallic strips or ribbons II and I2 separated throughout their length by a strip of insulating material I2a. When the parts are assembled, the rear ends of ribbons II and I2 are clamped in'positionby the yoke parts III, while the front ends of ribbons II and I2 are held by a U-shaped clamp I3, there being suitable insulating material It between the ribbons II. and I2 and the frame parts 8 and I and clamp I3. As shown, clamp I3 is mounted on one end of a rectangular or square shaft I5. An arm I6 is secured to the other end of shaft I5 and has three angularly offset portions such that the free end maycarry a stylus I'I adapted to ride-in the groove of a recording. The arm I9 may be constructed in accordance with the disclosure in my copending application Serial No.

- 390,822, filed April 28, 1941. A resilient bushing I8 of rubber or the like surrounds shaft I5 and is clamped in the yoke parts 9 to substantially secure shaft I5 against longitudinal movement, while permitting slight rotational movement by deformation .of element l8. The bushing I8, with yoke parts 9, serves to restore the shaft I3 to its normal angular position whenever it is displaced therefrom. The clamping frames 6 and "I are secured together by screws or the like with the structures still to be described. The circuit comprises a radio receiver of the super-" heterodyne type, wherein a common oscillator' circuit is employed both as the local oscillator of the superheterodyne during radio receptions;v and as the phonograph pickup oscfllator 'duringz.

phonograph operation substantially as disclosed in the applicatipn' of R. B. Albright, Serial No;

397,986, filed June 13, 1941. The radio receivermay comprise a radio frequency amplifier 22, a-

detector-oscillator stage including the tube '23,

an intermediate'frequency amplifier 24, a second detector and A. V. C. stage 25, an audio frequency amplifier 26, and a loudspeaker 21. when the switches 28, 29 are in their left-hand position, the receiver is adapted for radio reception. When the switches 28, 29 are in the position shown,-the phonograph pickupbscillator tank coil 30 is connected to the oscillator rid and oscillator anode of the tube 23. The

coil 30 may betunedto resonate at a desired frequency -by a fixed condenser 3|, a small. ad-

. justabl'e condenser 32, and the pickup condenser 33, with which the present invention is particularly concerned. Coil 30 is preferably tuned to a frequency above or below the intermediate frequency at a point on the I. F. selectivity'clia'racteristic where the characteristic is linear over an appreciable range of frequencies. If new the frequency of the phonograph oscillator is caused to vary about its mean frequency, due to the actuation of condenser 33,- the radio frequency output of the I. F. amplifier will vary in amplitude, and the output voltage of the second detector will vary in accordance with the audio frequency signal of the recording. This mode of operation of the receiver circuits is more fully described in the said Albright application and forms no part of the present invention.

Preferably a constant capacity cable, such as the coaxial cable 34, is employed to connect the pickup condenser 33 to coil 30. Since the capacity variations of condenser 33 are very'small, it is highly desirable that there shall be no extraneous capacity ing leads.

As mentioned hereinbefore, a capacity-type pickup such as shown in Fig. 2 is an amplituderesponsive device and, therefore, its output decreases with increasing frequency over the constant velocity portion of the recorded frequency range, thus attenuating the high frequency sounds and comparatively emphasizing the low frequency sounds. response of the pickup could be compensated by an electrical frequency-responsive correcting means in the circuit, but this would involve considerable complication and expense. I have found that the desired frequency compensation may be obtained in the pickup itself by varying the cross-section of the condenser plates, so as to vary the capacity changes according to the frequency. Preferably this is accomplished by suitably tapering the plates, as described more fully hereinafter, but the invention contemplates any suitable method of varying the plate cross-section for the stated purpose. In addition, I have discovered that the operation of the pickup may be improved by in presently to be described.

In Fig. 4 is illustrated one form of the capacitytype pickup constructed according to the present invention. This device preferably comprises three metallic strips or ribbons 35, 36 and 3'! .of phospher bronze, brass or some similar resilient material. These strips are separated throughout their length by thin layers of insulating material 38, for example-mica, and are embraced by a U-shaped clamp 39, similarly to the construction already described. Clamp 33 is mounted on one end of a short shaft 40, on the other end of which is carried the stylus 4|. If the clamp 39 is trically connect the two outer strips together, as shown in Fig. 4. A round resilient bushing 42 of rubber or the like surrounds shaft 40, and is adapted to co-operate with the semi-cylindrical bearing surfaces 43 of frame members 44 and 45. The parts are clamped together in a manner similar to that already described. In this instance, however, the outer plates 35 and 33 form one electrode of the condenser and are directly engaged by clamp 33 and frame members and 45. For a given plate area and needle swing, three plates result in twice the capacity variation of two plates and are accordingly preferred. However, it is within the scope of the invention to variations due to the connect This undesired frequencyv embodying other features there- 7 of metal, it may be used to elec provide a two plate condenser, in

which case the arrangement will be similar to Fig. 2.

In accordance with the present invention the plates 3!, 33 and 31 are of predetermined tapered form, as best shown in Fig. 6, in order to provide frequency compensation as mentioned above. It is also preferred to impart a set pretwist to the center plate about its longitudinal axis before assembly of the parts, as indicated at 46 in Fig. This imparts a certain amount of twist to the assembled elements. Practice has shown that best results are obtained by imparting a 20 twist to the center plate, after which the three plates are clamped together, the resulting assembly having a set pretwist of somewhat less than 20, which imposes no load on the bearing formed by the round bushing 42 and co-operating frame members 44 and 45. As shown in Fig. '7,- the two outer plates are bowed along their major axis in a direction outwardly from the center plate. This is done to insure more intimate contact of the plates and the di-electric after the assembly is clamped, as will be explained more fully hereinafter.

Although a variety of different plate dimensions and degrees of taper have been found to substantially improve the response above the turnover frequency, that is, level out the response characteristic throughout the constant velocity, diminishing amplitude range, the following specifications have accomplished the best results and are therefore given by way of example. The length of each plate element is 1% inches, the width at the wider clamped end being 1: inch, and the width at the narrower end, i. e that end embraced by the clamp 39, being a; inch. The plates are given an arcuate taper struck on a 4 inch radius, the center of said radius lying on a line drawn perpendicular to the major axis of the plate at the narrower end thereof. Flat, phospher bronze, sheet stock of 0.004 .inch thickness is preferably employed for the plates, while a mica sheet of' 0.0015 inch thickness is used for the dielectric. As pointed out above, the plate dimensions may be varied widely, and will still result in improved overall frequency response, as long as the moment of inertia about the longitudinal axis'of the plate is diminished,'as may be done, for example, by giving the plates 9. straight instead of an armate taper, or by punching a hole toward the free end of a rectangular plate. This reduction in cross-sectional area serves, in effect, to provide a plate having varying resonant frequency areas, with increasing frequencies as the narrower end of the plate'is approached. For this reason, a high frequency, low amplitude needle swing will move the plates at a frequency at which they will resonate very readily, and thereby result in a proportionately higher capacity change than results when a greater plate area is moved at or near the lower resonant frequency of the entire system, as will result when the plates are driven by a high amplitude, low frequency bass note. In effect, this invention provides condenser plates having a broad resonant period, with a progressively increasing resonance characteristic as the high audio frequencies are approached. An arcuate tape'r has been found preferable to a straight taper as it results in more even compensation for the diminishing amplitude characteristic above the turnover frequency.

The results obtained, employing the preferred bly will be venient point in the amplifyingcircuit, and for this reason absolute values are not given. The curve A is that obtained from the use of the rectangular plates of Fig. 2, while the curve marked B shows the results obtained with the improved condenser pickup of the present invention.

gees,

.Curve A exhibits considerable high frequency vides means for eliminating plate "breakup." and the consequent undesirable high level points occurring throughout the response. This is accomplished in the preferred forms shown in Figs. 4, 6 and 7, by bowing of the outer plates before assembly, which has the efi'ect of maintaining the plates in more intimate contact with the dielectric, as well as damping them within the operating range by increasing the rigidity of pickup device adapted for use in the reproduction of constant velocity records, a condenser formed of relatively movable flexible metallicelements having a fixed end and a free end and being adapted to be torsionally flexed about their longitudinal axes in response to the undulations of the record groove, said elements having a smaller lateral dimension at their free end than at their fixed end, whereby compensation is provided for the diminishing amplitude characteristic of a record recorded at constant velocity.

2. In an amplitude-responsive phonograph pickup device adapted for use in the reproduction of constant velocity records, a condenser formed of relatively movable flexible metallic elements having a fixed end and a free end and bethe assembly. Care should be taken that the rigidity is not increased to such a point that the amplitude of movement becomes too low, or the capacity change will be so small as to make the device impractical.

In Fig. 5 is shown an alternative way of avoiding such local undesirable resonance areas resulting from plate breakup." In this form each of the outer plates is provided with a rib extending longitudinally along the outside surface thereof, one of said ribs being indicated at 51 in Figs. 5 and 5a of the drawings. This form has the advantage of increasing the contact pressure along the edges of the plates, by reason of the slight lateral bowing which the ribbing imparts thereto. This is clearly seen in Fig. 5a..

The invention further provides means for trimming the condenser so as to sharply limit the high frequency response at any predetermined point. This is accomplished by varying the effective length-of the condenser plates by means of the small flat plate 48 shown in Fig. 5. A portion of this'rigid plate extends beneath and is adapted to be clamped by the frame members 44 and 45, when the device is assembled. If the member 48 is extended further out over the upper condenser plate, the effective length of the assemshortened and the high frequency cutofi raised. Similarly, the cutoff point may be lowered by moving the member 48 further beneath the frame members 44 and 45, and thereby increasing the effective length of the condenser assembly.

From the above description it will be evident mg adapted to be torsionally flexed about their longitudinal axes in response to the undulations of the recordgroove', said elements being of tapering plan form with their greatest lateral dimension toward said fixed end, to provide compensation for the diminishing amplitude 011318.61 teristic of a record recorded at constant velocity.

3. A construction in accordance with claim 2, wherein the tapering edges of said elements are arcuate in shape.

4. In an amplitude-responsive phonograph pickup device adapted for use in the reproduction of constant velocity records, a stylus, and a condenser formed of movable metallic elements, said condenser being mounted for movement in response to the vibrations of said stylus, and said elements being of generally tapered plan form to provide compensation for thediminishing. amplitude characteristic of a record recorded at constant velocity.

' 5. In an amplitude-responsive phonograph pickup device adapted for use in the reproduction of constant velocity records, a stylus, and a condenser formed of movable metallic strips, said condenser being mounted for movement in repensate for the diminishing amplitude characterspouse to the vibrations of said stylus, and the sides of said strips being of arcuate tapered shape, whereby the moment of inertia of various lateral sections taken about the longitudinal axis of a strip becomes progressively smaller as one end of said strip is approached.

6. In a-phonograph pickup device, a pair of oppositely facing frame members, a condenser comprising a plurality of resilient metallic elements supported by said-frame members for torsional deflection about an axis, and a stylus aflixed to said elements at one end of said axis, said elements being of diminished cross-section toward the stylus end of said axis, whereby to comistic of a record recorded at constant velocity.

7. In a phonograph pickup device, a pair of oppositely facing frame members each having a central opening bounded at opposite ends by yoke portions, a condenser comprising a plurality of that thls invention provides an amplitude-responsive, improves the fidelity of reproduction of constant velocity recordings, and in which the amount capacity-type transducer which greatly and range of response are very readily control- 1. In an amplitude-responsive phonograph resilient metallic elements each having one end rigidly clamped between one pair of said yoke portions, a resilient bearing secured between the other pair of said yoke portions, 9. vibratory shaft extending through said bearing, said shaft being secured to the other ends of said metallic elements, and a stylus aifixed to said shaft to torsionally deflect said elements about an axis, said elements being of diminished cross-section toward the stylus end of said axis, whereby to com-- pensate for the diminishing amplitude characteristic of a record recorded at constant velocity.

8. In an electro-acoustic transducer, a variable condenser responsive to torsional vibration about one axis thereof, one element of said condenser having a set pretwist; of predetermined amount about said axis, and means for fastening the condenser elements together whereby said condenser has a set pretwist of an amount determined by the degree of pretwist of said one element.

9. In an electro-acoustic transducer, a variable condenser comprising a plurality of resilient plate-like elements, an inner one of said elements having a set pretwist about one axis thereof, means for maintaining said elements in substantial conformity to the pretwist of said one element, and means for torsionally deflecting said elements about said axis.

10. In an electro-acoustic transducer, a variable condenser comprising a plurality of resilient strip-like members adapted to be torsionally deflected to vary the capacity of said condenser, at least some of Said members being prebowed to maintain the assembled members in closely spaced relation throughout their lengths, and means for fastening all of said members together at their ends.

11. A method of constructing a condensertype electro-acoustlc transducer, which comprises imparting a predetermined set pretwist to one of the condenser elements prior to assembly thereof, bowing other of the condenser elements prior to assembly thereof, and clamping the condenser elements at spaced points during assembly thereof, whereby the assembled elements are affected by the pretwisting and bowing of the elements.

12. A method of constructing a condenser-type electro-acoustic transducer employing fiat resilient strips as condenser elements, which comprises imparting a predetermined set pretwist to one of said strips about its longitudinal axis prior to assembly of the condenser elements, bowing others of said strips longitudinally prior to assembly of the condenser elements, and clamping the ends of said strips during assembly of the condenser elements, whereby the assembled strips are caused to assume desired positions in the assembled structure.

13. A method of constructing a condenser-type electro-acoustic transducer employing two outer resilient strips and an intermediate resilient strip as condenser elements, which comprises imparting a predetermined-set pretwist to said intermediate strip about its longitudinal axis prior to assembly of the condenser elements, bowing said outer strips longitudinally prior to assembly of the condenser elements, and clamping the ends of the strips during assembly of the condenser elements, whereby the assembled strips are caused to assume desired positions in the assembled structure.

14. In an electro-acoustic transducer, a vari able condenser comprising a plurality of resilient strip-like members adapted to be torsionally deiiected to vary the capacity of said condenser, and means for maintaining said members in closely spaced relation throughout their lengths, said means comprising a strengthening rib extending longitudinally of at least one of said strip-like members.

15. In a phonograph pickup device including a variable condenser comprising. a plurality of flexible fiat metallic members arranged in superposed relation, and means including a vibratory stylus arm adapted to torsionally deflect said members to vary the capacity of the condenser, certain of said members being of non-planar form such that adjacent members are maintained in closely spaced relation throughout their lengths during torsional deflection thereof.

16. In a phonograph pickup device, a variable condenser comprising flexible metallic members, means adapted to vibrate said members to vary the capacity of said condenser, and adjustable means for varying the effective length of said members to alter the frequency response characterlstic of said pickup.

17. In a phonograph pickup device, a variable condenser assembly comprising flexible metallic members clamped at one end, means operatively associated with the opposite end of said assembly adapted to vibrate said members to vary the capacity of said condenser, and means comprising a rigid member extending over said assembly from a region adjacent the clamped end of said members and contacting one of said members, the degree of extension of said rigid member being adjustable to alter the frequency response characteristic of said pickup.

FREDERICK R. FARROW, JR. 

